1. Field of the Invention
The present invention relates to a differential amplifier made of a semiconductor material and particularly to a differential amplifier having a wide sensitivity range.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a differential amplifier according to a first embodiment of the prior art. FIG. 2 is a circuit diagram showing a simplified circuit of the differential amplifier of FIG. 1. FIG. 3 shows voltage-current characteristic curves of the circuit of FIG. 2.
FIG. 4 is a circuit diagram showing a differential amplifier according to a second embodiment of the prior art. FIG. 5 a circuit diagram showing a simplified circuit of the differential amplifier of FIG. 4. FIG. 6 shows voltage-current characteristic curves of the circuit FIG. 5.
The differential amplifier of the first prior art embodiment of FIGS. 1 to 3 will be explained.
In FIG. 1, MOSFETs Q1 and Q4 form a pair of inverters, and MOSFETs Q2 and Q5 another pair of inverters. A MOSFET Q3 is a constant current MOSFET. For the sake of simplicity of explanation, the MOSFET Q3 of FIG. 1 is removed in FIG. 2 with reference to which a differential gain and an offset of the differential amplifier of the first prior art embodiment will be explained.
When a voltage V0 is applied to input terminals IN1 and IN2 in FIG. 2, an operating point of the differential amplifier will be a point M in FIG. 3.
When .+-..DELTA.V is applied to the input terminals INl and IN2, output terminals OUT1 and OUT2 provide a potential difference represented with points L and H in FIG. 3. This potential difference is called the differential gain.
When a voltage of V0+.DELTA.V is applied to the input terminals IN1 and IN2, a voltage at each of the output terminals OUT1 and OUT2 shifts from the operating point M to an operating point M'. When a voltage of V0-.DELTA.V is applied to the input terminals IN1 and IN2, a voltage at each of the output terminals OUT1 and OUT2 shifts from the operating point M to an operating point M". This is called the offset, which indicates a shift of the operating point.
In this way, according to the differential amplifier of the first prior art embodiment, an increase in the differential offset inevitably accompanies an increase in the gain, so that this differential amplifier achieves a narrow sensitivity range.
The differential amplifier of the second prior art embodiment will be explained with reference to FIGS. 4 to 6.
In FIG. 4, MOSFETs Q1 and Q4 form a pair of inverters, and MOSFETs Q2 and Q5 form another pair of inverters. Each pair of the inverters has a CMOS configuration. For the sake of simplicity of explanation, a MOSFET Q3 of FIG. 4 is removed in FIG. 5 with reference to which a principle of the second prior art embodiment will be explained.
When a voltage V0 is applied to input terminals IN1 and IN2 in FIG. 5, an operating point of the differential amplifier will be a point M in FIG. 6.
When .+-..DELTA.V is applied to the input terminals IN1 and IN2, output terminals OUT1 and OUT2 provide a potential difference represented with points L and H in FIG. 3. This potential difference is the differential output.
When the input voltage is changed from V0 to V0+.DELTA.V, the operating point is shifted to a point M', and when the input voltage is changed to V0-.DELTA.V, the operating point is shifted to a point M".
Since the differential amplifier of the second prior art embodiment has the inverter pairs of CMOS configuration, it may provide a large differential gain but with a large offset. Namely, this differential amplifier achieves a very narrow sensitivity range.
In this way, the conventional differential amplifiers may provide a large differential gain but with a large offset which drastically narrows a sensitivity range of the amplifiers.